The present invention relates to a vehicle braking system which can produce a braking force automatically under predetermined conditions and, particularly to a vehicle braking system including a servo booster connected to a brake pedal of the vehicle, a hydraulic braking circuit connected to the servo booster, and a booster control device for generating an output force on the servo booster independently of the actuation of the brake pedal.
When the driver of the vehicle wishes to maintain the stopped condition of the vehicle on a sloping road or on a crowded road, it is desirable that the braking force be able to maintained even though the depressing force on the brake pedal is released. One prior art braking system generates a braking force automatically upon detecting that the vehicle speed is nearly zero and that the gas pedal is not depressed and releases the braking force automatically upon detecting that the gas pedal is depressed. Further, such braking system automatically releases the braking force when the transmission lever is shifted to "reverse" position so as to enable the driver of the vehicle to drive the vehicle at a very low speed without depressing the gas pedal while shifting the vehicle rearwards such as driving the vehicle into a garage. However, there is a drawback such that when the braking force has been applied automatically with the vehicle being stopped on a downwardly sloping road, and the driver wishes to rearwardly drive the vehicle and shifts the transmission lever to "reverse" position, the braking force is released automatically and the vehicle moves forward.
Further, it is desirable to control the braking force such that the braking force is automatically reduced in response to the decrease in the speed of the vehicle being braked so as to achieve smooth stoppage of the vehicle and to prevent the so-called swing-back phenomenon accompanying the sudden stoppage of the vehicle. In one prior art braking system, the braking force is decreased to a predetermined low level when the speed of the vehicle being braked is decreased to a predetermined low speed. In another prior art braking system, the braking force is adjusted automatically such that the change in the speed of the vehicle being braked in a predetermined time interval falls within a predetermined range. However, in the former system, since the decreased braking force is constant, the adjustment for achieving optimum braking effect on a level road is not satisfactory on a sloped road and, in the latter system, since the adjusted range in speed change of the vehicle is a constant, the adjustment for minimizing the swing-back phenomenon at the time of stoppage inevitably elongates the braking distance.
Further, in a still further prior art braking system, the braking force which has been applied automatically is released in response to the actuation of a parking brake. However, there is a shortcoming that the automatic braking force is sometimes released too early before a sufficient braking force is achieved by the parking brake whereby the vehicle moves inadvertently.